1. Field of the Invention
The present invention relates to a video cassette tape recorder of high picture quality, and more particularly to a picture control system for use in a video cassette tape recorder which can reproduce a precise picture suitable for a high resolution by simultaneously executing hard-control and soft-control of a reproduction luminance signal by separate circuits.
2. Description of the Prior Art
Various types of picture control systems for use in a video cassette tape recorder are well known in the art. One conventional picture control system for use in a video cassette tape recorder is illustrated in FIG. 1. As shown in FIG. 1, the conventional picture control system comprises a luminance signal input section 101, including a resistor R101 and transistors TR101-TR103, for amplifying and outputting a reproduction luminance signal YS at a predetermined level, a low-pass filter 102, including of a resistor R102 and a capacitor C101, for filtering an output of the luminance signal input section 101, a differential amplifier 103, including resistors R103-R111 and transistors TR104-TR111, for differential-amplifying and adding together the reproduction luminance signal 45 of the luminance signal input section 101 and 94 output signal of the low-pass filter 102, a bias varying section 104, including of a resistor R112, a capacitor C102 and a variable resistor VR101, for varying a bias voltage of the differential amplifier 103, and a signal compensating section 105, including resistors R113-R119 and transistors TR112-TR120, for adding the output signal of an differential amplifier 103 to the reproduction luminance signal YS and then amplifying and outputting the added signal. In FIG. 1, references BV1-BV4 denote bias voltages which are determined by power supply Vcc and value of each of the circuit elements.
Such a conventional picture control system operates as follows. When a power Vcc is supplied and bias voltages BV1-BV4 are supplied, transistors TR101, TR113 and TR115 become conductive, respectively, by the bias voltages BV1, BV2 and BV3 and transistors TR103, TR110, TR111, TR116, TR117 and TR120 are also turned on by the bias voltage BV4. Furthermore, the power Vcc is divided at the resistors R103 and R104 and applied to bases of the transistors TR104 and TR107 via the resistor R105 as a bias voltage and also applied to bases of the transistors TR105 and TR106 via the resistor R106 as a bias voltage. At this moment, the bias voltage which is applied to the transistors TR105 and TR106 is varied in response to the variable resistor VR101 of the bias varying section 104.
Under these circumstances, when a reproduction luminance signal YS as shown in FIG. 2A is inputted, the reproduction luminance signal YS is applied to a base of the transistor TR102 of the luminance signal input section 101, thereby a signal in proportion to the reproduction luminance signal YS is outputted from an emitter of the transistor TR102. And, the outputted signal is applied to a base of the transistor TR109 via a resistor R108 of the differential amplifier 103 and filtered at the low-pass filter 102, as shown in FIG. 2B, and then applied to the base of the transistor TR109, Accordingly, the transistors TR108 and TR109 are differential-amplified so that only a high frequency component is output to their collectors as a current, and this output current is properly amplified at the transistors TR104 and TR107 and then output to an output terminal VO1. Meanwhile, the bias voltage which is applied to bases of the transistors TR105 and TR106 is varied in response to the change of the variable resistor VR101 of the bias varying section 104, and at this moment, when the bias voltage is so adjusted as to execute a hard-control operation, a signal having the waveform of FIG. 2C is output from the output terminal VO1.
The signal outputted as above is applied to a base of the transistor TR112 and output through its emitter and then applied to an emitter of the transistor TR115 through a resistor R113. And, at this moment, the reproduction luminance signal YS, as shown in FIG. 2A, is applied to a base of the transistor TR114 and output through its emitter and then applied to an emitter of the transistor TR115 via a resistor R115. As a result, since the signal of the output terminal VO1, as shown in FIG. 2C, is added to the reproduction luminance signal YS, as shown in FIG. 2A, a signal, as shown in FIG. 2D, is output. That is, the reproduction luminance signal YS is outputted in a state that its leading edge and trailing edge are emphasized.
In the meantime, under the state that the bias voltage which is applied to the bases of the transistors TR105 and TR106 is so adjusted as to execute the soft-control operation, a signal having the waveform of FIG. 2E is outputted from the output terminal VO1, and since this signal is added to the reproduction luminance signal YS, as shown in FIG. 2A, at the signal compensating section 105, a signal, as shown in FIG. 2F, is outputted from the output terminal VO, and thus the reproduction signal is soft-compensated and then outputted.
In such a conventional system, since the hard-control or soft-control operation is carried out by the change of the bias varying section and compensation is executed only in one direction at the edge portions of the reproduction luminance signal at the time of hard-control or soft control, there exists a limit to the compensation of the reproduction luminance signal and also it is difficult to obtain a precise picture control.